In the frame and phase synchronizing apparatus disclosed in commonly assigned published German patent application DE-OS No. 32 27 151, a received character stream is sampled at a sampling rate set by a sampling clock. These sample values are digitized by means of an analog-to-digital converter; transmission distortions in the original digital character stream remain in the digitized sample values. A digital correlator forms the cross-correlation function from a sequence of digital sample values and a known unique word stored in the receiver which serves as a frame synchronization word. The cross-correlation function attains a maximum value when the unique word appears in the received character stream. From these maxima, the frame clock is derived. Only isolated sample values of the cross-correlation function are obtained by the time-discrete sampling process. However, from the two sample values of the cross-correlation function which respectively precede and follow the maximum sample value, it can be determined whether those two values are symmetrically disposed in time (ie, centered) about the maximum value. Clock phase control information can thereby be derived which is used to adjust the phase of the sampling clock.
In the above referenced apparatus, the phase of the sampling clock has to be adjusted slowly. Since any distortion in the character stream will also be present in the individual samples, the values used to derive the control information will include components which originate from pre-oscillations of character data following the unique word and from post-oscillations of character data preceding the unique word. In the prior art, such components are filtered out by averaging over several frame periods.
Both the frame clock and the character clock are thus derived from the received unique word. During transmission along a transmission path, the unique word is distorted in a particular manner. If this distorted unique word is sampled with a phase-error-containing sampling clock, the phase error can be determined from the sample values. However, as part of the distortion on the transmission path, pre-oscillations and post-oscillations are superimposed on the unique word caused by character data respectively following and preceding the unique word, thereby complicating the evaluation of the latter. In the above-referenced prior art apparatus, such pre-oscillations and post-oscillations are taken into account by averaging over several frames, thereby slowing the clock synchronization process.